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Chapter 9

To familiarize you with Simple PERFORM How PERFORM statements are used for

iteration Options available with PERFORM

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Simple PERFORM Reviewed Iteration Using Other Types of PERFORMs Using Nested PERFORM VARYING

Statements PERFORM WITH TEST AFTER option

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Some new stuff

PERFORM [paragraph-name-1]

Executes all instructions in named paragraph

Then transfers control to instruction following PERFORM

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Format

Use to execute a paragraph from different points in a program

Use to modularize program◦ Write each set of related instructions as separate

module or paragraph◦ Use PERFORM paragraph-name to execute each

module as needed

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PERFORM...

END-PERFORM Use when only a few statements are to be

executed Modularize with PERFORM paragraph-name

when many statements required

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Statements to be executed

Format

PERFORM may be one instruction in range of another PERFORM

Perform 200-Paragraph .

.

200-Paragraph. Perform 500-Paragraph

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Nested PERFORM

In-line PERFORMs can include nested in-line PERFORMs or PERFORMs with paragraph-name

Perform. . .

Perform . . .

End-Perform. . .

End-Perform

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PERFORM paragraph-name-1

THROUGH

THRUparagraph-name-2

Use expanded format to execute all statements, including other paragraphs, from paragraph-name-1 through paragraph-name-2

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Format

GO TO paragraph-name-1

Does not return control to following statement like PERFORM

Transfers control permanently to another paragraph

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Format

Sequence◦ instructions executed in order in which they

appear IF-THEN-ELSE or selection

◦ instructions executed depending on value of condition

Iteration or looping◦ series of instructions executed repeatedly◦ either in-line or in different module

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PERFORM [paragraph-name-1]UNTIL condition-1

Repeats statements in paragraph until condition is true

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Format

No paragraph name follows PERFORM Instead statements to be repeated placed

between PERFORM UNTIL … END-PERFORM

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Often want to perform some action a certain number of times

Use a field as a counter to count number of times action is repeated

Set field to zero initially, then increment it by 1 each time action repeated

When field equals number of times action is to be repeated, condition is met and loop ends

Stuff you know already

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Count initialized to zero so not equal to 3 when condition checked first time

Hello displayed on screen and Count incremented to 1

Move Zeros To CountPerform Until Count = 3

Display 'Hello'Add 1 To Count

End-Perform

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Precede loop by instruction to initialize field to be tested

Include PERFORM UNTIL …that repeats until field tested reaches desired value

Include instruction in loop to change value of field tested so that condition is eventually met

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Condition tested before paragraph or in-line statements executed even once

If condition met on first test, paragraph or statements executed zero times

Move 6 To XPerform 300-Process-Rtn

Until X > 5

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Example Paragraph executed 0 times

Loop stops when condition is true One of instructions in loop should change

identifier used in condition

Move 0 To YPerform Until Y > 10

. . .Add 1 To Y

End-Perform

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Example

Changes Y so condition eventually met

Executes a sequence of steps a fixed number of times

No counter needed Loop below executes paragraph 300-Print-

Rtn 5 times

Perform 300-Print-Rtn 5 Times

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May use field whose value represents number of times to repeat loop

Field must be numeric, containing only positive integers or 0

Loop below performs 300-Print-Rtn ten times

Move 10 To How-ManyPerform 300-Print-Rtn How-Many Times

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Also used with in-line loop Loop below executes MULTIPLY statement 3

times

Move 2 To NumPerform 3 Times Multiply 2 By NumEnd-Perform

Num equals 16 when loop ends

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Sum even integers from 2 through 10 Initialize a field to first number to be added

(2) Increment field by 2 so it equals even

numbers (2, 4, 6, 8, 10) Use this field's value to

◦ Test in condition◦ Add to a total field to find sum

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Sum even integers from 2 through 10

Move 0 To TotalMove 2 To CountPerform Until Count > 10Add Count To TotalAdd 2 To Count

End-PerformDisplay 'Total=', Total

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Initialize field to be tested

Test field until it reaches desired value

Change field tested so condition eventually met

Result: Total = 30

Perform 5 TimesPerform 10 Times

Read record from file and add its amount to group

totalEnd-Read

End-PerformPerform Print-Group-Total

End-Perform

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Outer loop

Inner loop

Use PERFORM … TIMES if you know in advance the number of times loop statements are to be executed

Use PERFORM … UNTIL if number of times loop repeated is needed for output or calculations

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PERFORM VARYING identifier-1 identifier-2 identifier-3

FROM BYinteger-1 integer-2

UNTIL condition-1

statement-1 …

END-PERFORM

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Format

Repeatedly executes statements in loop while varying value of a field

First identifier-1 is given FROM value Condition then tested Executes statements in loop if condition not

met Then adds BY value to identifier-1 and

repeats condition test

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Perform Varying Ctr From 1 By 1Until Ctr > 5Display 'Ctr = ', Ctr

End-Perform

Sets Ctr to 1, since Ctr > 5 not true, executes DISPLAY statement

Increments Ctr by 1, tests condition again Like the for next loop in C with a step increment

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Finds sum of odd numbers from 1 to 25Move 0 To TotalPerform Varying Ctr From 1 By 2

Until Ctr > 25Add Ctr To Total

End-PerformDisplay 'Total = ', Total

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Output: Total = 169

Statements to be repeated may also be in separate paragraph

Perform 300-Process-Rtn

Varying Ctr From 1 By 1 Until Ctr > 20

Executes 300-Process-Rtn 20 Times

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May include a PERFORM VARYING loop as one of statements in another PERFORM VARYING loop

Each time outer loop is repeated, inner loop is executed until its condition is met

Following example prints the times tables for numbers 1 to 9

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Perform Varying N1 From 1 By 1Until N1 > 9Perform Varying N2 From 1 By 1

Until N2 > 9Compute Product = N1 * N2Display N, ' * ' M ' = ',

ProductEnd-Perform

End-Perform

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Outer loop

Inner loop

Condition tested before statements in loop executed first time

If condition met on first test, statements not executed at all

Can specify that condition be tested after instructions executed first time

Then instructions always executed at least once

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PERFORM [paragraph-name-1] BEFORE

[WITH TEST ] AFTER

UNTIL condition-1

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Format

Perform With Test AfterUntil Opt-Num >=1 And <= 5Display 'Select option (1-5)'Accept Opt-Num

End-Perform

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Example

Formats of PERFORM Statement◦ Simple PERFORM

In-Line PERFORM PERFORM … END-PERFORM

PERFORM paragraph-name-1 [THRU paragraph-name-2]

Causes execution of instructions in named paragraph(s)

After paragraph executed, control returned to statement after PERFORM

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Formats of PERFORM Statement◦ PERFORM UNTIL repeats instructions until a

condition is met Condition may be tested before or after instructions

are executed◦ PERFORM … TIMES

Use when you know exact number of times loop statements are to be executed

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Formats of PERFORM Statement◦ PERFORM VARYING

Automatically initializes and changes value of loop counter

Nested PERFORMS (PERFORM statements within PERFORM statements) allowed

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In-line PERFORMs permitted with all PERFORM options◦ Code does not need to be in separate paragraph◦ Terminated with END-PERFORM

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